Photothermal catalysis is an innovative approach that combines typical photocatalysis and traditional thermocatalysis, which possesses all the merits of both processes, such as the efficient catalytic rate of thermocatalysis, the low energy consumption, low pollution and high selectivity of photocatalysis. Meanwhile, photothermal catalysis avoids the problems of single approach like the high temperature of thermocatalytic reaction, a series of side reactions, and deactivation of catalysts. Furthermore, the efficient solar energy utilization rate of photothermal catalysis holds the promise of elevating reaction rates to industrial levels. Due to its exceptional solar energy utilization efficiency, high catalytic reaction rates, mild reaction conditions, and low pollution, photothermal catalysis has been proposed as a promising alternative to traditional photocatalysis and thermal catalysis in the fields of energy conversion and environmental remediation.

This review first introduces the mechanism of photothermal catalysis, with a focus on the plasma conversion and non-plasma conversion processes. On the basis of synergistic modes between photocatalysis and thermocatalysis, we categorize photothermal catalysis into three types: photo-assisted thermocatalysis, heat-assisted photocatalysis and photothermal co-catalysis. Subsequently, various types of photothermal materials, including metals, semiconductors, carbon-based materials and metal-organic frameworks are summarized. Thereafter we comprehensively discuss the current effective strategies to improve the photothermal performance, such as improving solar energy absorption through band engineering and morphological manipulation, increasing carrier separation efficiency via heterojunction construction, and enhancing thermal management through heat insulation, suppression of infrared radiation, and thermal energy storage. Lastly, the future research directions and challenges in photothermal catalysis is also discussed.

Photothermal catalysis, based on the photothermal conversion to drive the reaction, shows large potentials in various applications like CO₂ reduction, organic pollutant degradation, organic synthesis, and hydrogen production via water splitting is the synergy of photochemical and thermochemical effects. This review summarizes the mechanisms of photothermal catalysis, including the direct process by infrared radiation absorption and the indirect process via non-radiative carrier relaxation. Moreover, the indirect way could be further divided into plasma conversion and non-plasma conversion. Based on the contributions of photocatalysis and thermocatalysis, photothermal catalysis is classified into three modes: Photo-assisted thermal catalysis, thermal-assisted photocatalysis, and photothermal co-catalysis. Metals, semiconductors, carbon-based materials and metal-organic frameworks are common photothermal materials. Various modification strategies are developed to promote the catalytic reactivity of photothermal materials, including improving solar energy utilization, accelerating the separation rate of high-energy charge carriers, and strengthening the thermal management. Though photothermal catalysis shows great advantages in comparison with the traditional thermocatalysis and common photocatalysis, it still faces a series of challenges especially in large-scale applications. Several research directions could be considered to the development and application of photothermal catalysis in the future.

The development of efficient, cost-effective and robust photothermal materials is the central theme in photothermal catalysis. At present, common photothermal materials focus on metals with LSPR effect, semiconductors and carbon-based materials. Emerging two-dimensional materials such as phosphorene, borene, MXene are capable of harnessing both visible and infrared light and also exhibit high photothermal conversion efficiency. These materials hold great potential as light absorbers in photothermal catalytic systems. Additionally, covalent organic framework materials with large conjugated systems also provide a new avenue for the development of high-efficiency photothermal catalysts.

The mechanism of photothermal catalysis need to be further clarified. The understanding of the insight of reaction could guide the construction of photothermal catalytic system. At present, the key research focuses on the synergestic effect between high-energy carriers and photo-derived thermal energy. More details about the exact energy conversions like light to charge carriers or non-radiative relaxation could be figure out. Furthermore, it is pivotal to distinguish the contribution of photocatalysis and thermocatalysis in photothermal catalytic reaction. The rational design of experiments, specific characterizations like in-situ techniques-including in-situ Raman, XPS, AFM, etc. and first principle calculations might provide the possible solutions to discern the catalytic mechanism.

To address the limitations of photothermal catalysts, it is highly desirable to explore simple and effective modification strategies to enhance the absorption and conversion of full-spectrum solar energy. Rational structural modulation of photothermal catalysts is essential to ensure efficient separation of the generated high-energy carriers and then apply to the subsequent reaction. Additionally, reducing the heat loss during photothermal catalytic processes is also a critical research priority. This requires balancing the energy utilization and heat dissipation while optimizing reaction pathways to achieve energy-efficient and sustainable catalytic systems.

The application of photothermal catalysis still requires further investigation, particularly in CO₂ reduction and C–C coupling to generate high-value C₂₊ products with high selectivity, which are the critical approaches to achieve the carbon neutrality.

C₃N₅ as a two-dimensional (2D) layered polymer material has great prospects in the field of energy storage due to the excellent light absorption, low electron transfer resistance, and environmental friendliness, etc. However, several drawbacks such as the high charge carrier recombination rate, weak reduction ability and low density of surface reactive sites give rise to a poor photoactivity, restricting the large-scale application. Recent researches focus on the synthesis and the detailed molecular structure of C₃N₅, as well as the various modification strategies to promote the photocatalytic activity. It is thus necessary to provide a general guidance for designing high-efficient C₃N₅ catalyst systems based on the existing results.

C₃N₅ is commonly prepared by thermal polymerization method in the presence of organic materials precursors that contains a large amount of nitrogen element like melem hydrazine, 3-Amino-1,2,4-Triazole, 5-Amino-1h-Tetrazol, etc.

C₃N₅ has three structures like triazine-triazole structure, azo structure and terminal triazole structure. The unique structures and bonding types of C₃N₅ endow it with a promising possibilities for photocatalytic applications.

Various modification strategies including morphology control, nitrogen vacancy creation, element doping and heterojunction construction in promoting the photocatalytic activity of C₃N₅ are discussed. Morphology control is beneficial to improving the specific surface area and enhancing the density of surface reactive site of C₃N₅. Nitrogen vacancy and element doping favors optimizing the band structure and improving the utilization of solar energy. Heterojunction construction like the Schottky junction, type-Ⅱ, Z-scheme and S-scheme C₃N₅-based heterojunctions enable the efficient spatial separation of charge carriers and maintain the intense redox capabilities.

C₃N₅ as a two-dimensional (2D) layered polymer material has advantages such as the unique structures, larger amount of nitrogen active sites, narrower band structure and excellent chemical stability. It is more favorable to the development prospects of C₃N₅ in various photocatalytic fields. This review summarizes the synthesis and molecular structure of C₃N₅, and the modification strategies developed to improve the photocatalytic performance of C₃N₅ nanomaterials in recent years, including morphology control, nitrogen vacancy creation, element doping and heterojunction construction. However, compared with the abundance of other semiconductor catalyst systems, a research on C₃N₅ still needs to be further explored in terms of rational preparation, construction of unique composite systems, elucidation of the intrinsic mechanism, and the application areas, etc.

In rational preparation, the eco-friendly and cost-effective large-scale preparation of C₃N₅ nanomaterials is still a challenge. The relationship between the reaction parameters in the thermal polymerization process and the resulting morphology structure is unclear, which hinders an ability to optimise and control the process further. Also, the synthesis of C₃N₅ inevitably results in the formation of toxic by-products, necessitating the development of supplementary follow-up treatment technology. The optimization of the rational synthesis of C₃N₅ for environmentally and large-scale preparation is more in line with sustainable development strategies.

In the construction of unique composite systems, the precise regulation is a pivotal concern, both in terms of industrial applications and fundamental scientific research. Until now, it becomes a challenge to elucidate the precise conformational relationship between vacancy/elemental sites density and photocatalytic performance, representing a significant obstacle to further performance optimization. The advancement of the precise regulation is instrumental in enhancing the catalytic performance of C₃N₅-based materials.

In the elucidation of the intrinsic mechanism, it is essential to investigate the atomic and electronic structures at the material/interface level in order to clarify the performance of the reaction process. This provides a theoretical foundation for the subsequent design and development of efficient photocatalysts. It is thus necessary to employ advanced characterizations like environmental transmission electron microscopy, aberration scanning transmission electron microscopy, synchrotron radiation and in-situ spectroscopy. The structural configurations for catalysts and the formation of intermediates during photoreaction can be monitored. The fine update is more beneficial to optimizing the properties of the catalyst material.

In the context of application areas,, the optimization of selectivity in catalytic reactions represents a crucial challenge for designing high-efficient photocatalytic systems. In the context of energy catalysis, such as methane conversion and CO₂/N₂ reduction, a detailed understanding of the intrinsic reaction mechanism and kinetic processes elucidated by theoretical simulation and in-situ monitoring is beneficial for the design and development of C₃N₅ catalysts with a high selectivity and a purpose of improving the yield of the target product. In the context of environmental remediation, the utilization of wastewater and seawater instead of purified water for hydrogen production from water is another way to achieve a sustainable development strategy. The design and development of C₃N₅-based materials with a high selectivity can facilitate their broader range of applications.